Over the period of its service life, a fully formulated motor oil deteriorates and becomes ineffective for protecting the moving and rubbing parts of an engine. Under normal operations, deterioration occurs mostly due to thermal break-down of the oil. In the early stages of service life, there exist adequate amounts of antioxidant and antiwear agents in an oil formulation. In simplified situations, the antioxidants provide protection for other constituents of the oil formulation, mainly the base oil, from thermal oxidation reactions, whereas the antiwear agents (zinc dialkyldithiophosphates (ZDDP) in most current motor oil formulations) protect the lubricated parts of the engine by chemically and/or physically being adsorbed on the surfaces of these parts to form a protective film. However, both the antioxidant and antiwear agents are consumed steadily as the service time increases.
As the service time increases, a point will be reached at which the level of the antioxidants in the oil formulation will be so low that they will no longer protect the oil from undergoing rapid oxidation reactions. Thereafter, the viscosity of the oil increases drastically over a very short period of time due to the massive formation of polymeric oxidation products. This will lead to engine damage if the oil is not changed immediately. Obviously, knowing the level of the antioxidants and its change in the oil is critically important for monitoring the oil quality during its service life.
Electrochemical methods are versatile, sensitive, and allow easy control of experimental conditions. Much effort has been executed to develop a simple but sensitive device which can be used to monitor deterioration of a motor oil in-situ. However, some of the techniques developed to date require chemical separations before an oil sample can be analyzed, are not very sensitive or have a slow response time.
An electrochemical method is reported by Richard J. Price et al., "Chemical Sensing of Amine Anti-Oxidants in Turbine Lubricants", Analyst, November, 1991, Vol. 116, pages 1121-1123. Price et al. disclose an electrical chemical sensor for oil formulations comprising a 3-electrode arrangement having a silver wire acting as a pseudo reference, a platinum wire functioning as a counter electrode and a platinum microdisc as a working electrode. The electrodes were coated with a polymer film cast from a solution of poly(ethylene oxide) and LiClO.sub.4 in acetonitrile-methanol.
Although the electrode was able to determine the presence of amine antioxidants in turbine lubricants, the electrode suffers from various disadvantages in that it is not a very good conductor, the response time is long and it is not very sensitive to antioxidant or antiwear concentrations.
Accordingly, a need exists in the art for an electrochemical sensor for motor oils which is sensitive and have a quick response time.